Automotive vehicle speed control systems must be capable of being immediately interrupted under certain operating conditions, for example upon operation of the brake, so that the speed control system does not tend to accelerate a vehicle if the operator wishes it to be slowed. Further, the system should not permit control of vehicle speed from a stopped or low-speed operating condition of the vehicle. Thus, the control system should be disabled below certain vehicle speeds. If the vehicle in which the control system is installed has a manually operated transmission, the control system should be disabled in the gear change interval to prevent excessive acceleration of the engine of the vehicle. Of course, it should also be possible to provide a manual disconnect switch.
The various conditions above referred to provide disconnect signals. A disconnect memory is provided so that, when one of the disconnect signals is effective, the control system will not be inadvertently enabled even by operation of a manual connecting switch.
The system should, additionally, be capable of interconnection with the remainder of the vehicle system, for example with the brake light switch, so that the system will be disabled if the brake light switch should become defective. Thus, if the actual vehicle speed differs from the commanded vehicle speed by a predetermined difference, control of the speed of the vehicle by the system should be inhibited. This last condition should, however, be capable of being overridden by a manually operated control switch, for example by a "resume" switch so that, after braking of the vehicle, the actually commanded speed can be reached automatically by the speed control system.
Control systems have previously been proposed in which the disconnect memory includes a relay having a switching contact connected in the current supply circuit of the positioning element for the engine, and which controls, for example, fuel supply thereto. The relay itself is controlled by the respective collector-emitter paths of various transistors, which in turn are controlled over appropriate threshold switches to which the various disconnect signals are applied. Upon conduction of the transistors, operation of the ON switch can energize the relay which is connected in a self-holding circuit for so long as current supply of the control element for the engine is closed. Operating the relay as connected requires a substantial number of components, which is expensive in costs as well as in assembly. Due to the relay, the system is subject to malfunction.